For disc brakes having one or more sliding discs the discs are normally received on a hub or other central part rotating in conjunction with the associated wheel. To simplify the description the term “central part” is used in a broad sense and is to be construed as covering any part receiving the brake disc(s), including a hub. The central part normally has splines, teeth or the like on an outer circumference for co-operation with teeth, splines or the like on an inner circumference of the brake disc. In that way the discs are rotationally fixed to the central part but may move in axial direction on the central part. On the same central part one or more discs may be received fixed in axial direction, while one or more discs are received moveable in axial direction. The contact faces of the teeth, splines or the like may be straight or arced.
As used in this description the expressions “radial”, “axial”, and similar expressions are in relation to the brake disc. Thus, “axial” is the direction of the rotational axis of the brake disc. “Tangential direction” as used here refers to the direction of rotation of the brake disc. Thus, the “tangential direction” is perpendicular to the “radial direction” as well as to the “axial direction”.
In a disc brake one or more brake pads and one or more brake discs are pressed against each other during brake actuation. The rotation of the central part receiving the discs and the rotation of an associated wheel are interconnected. Thus, the braking of the rotation of the brake discs will be transferred to a braking of the associated wheel.
During braking the connections between the central part and the discs are subject to mechanical strains caused by large braking forces, by heat etc.
The brake disc is considerably heated up during braking, due to the fact that it absorbs kinetic energy from the vehicle. The heating of the disc leads to a heat expansion. However, the disc may have different heat expansions due to uneven heating. The inner part, i.e. the part closest to the central part, may be much less heated than the main part, i.e. the rest of the brake disc. The lower heating of the inner part of the disc is often due to the teeth connection with the central part, which prevents this area from being effectively squeezed, swept and heated up by the brake pads. The uneven heating leads to tangential tension and possible crack formation in the brake disc.
The inner part of the disc having teeth provides uneven resistance against the forced tangential tension caused by the uneven heating of the disc. This gives an uneven distribution of the forced tension so that the material just radially outside the gaps will get an enlarged proportion of the total tangential tension.
Large bending stresses appear at the teeth roots at braking. If the brake is applied to produce a large braking torque when the brake disc has got the uneven temperature distribution described above, the bending stresses may add extra tension to the material close to and radially outside the gaps. This material is already heavily strained by the thermal expansion of the material in the main part of the disc and the extra tension may create a very high total tension of the material.